The present invention relates to stud welding apparatus and methods and, more particularly, to methods and apparatus for welding, by the stud end welding technique, a stud having a plurality of ends which are to be simultaneously welded to a workpiece.
The stud end welding technique has been known and in practical commercial use for many years. In this welding technique, a single end of a metallic member, such as a threaded bolt or the like, is welded to a metallic member generally known as a base member by the application of sufficiently high current passing through the stud and across an arc between the stud and the workpiece to create a molten pool of metal into which the stud is ultimately plunged and secured following solidification of the molten pool.
There are many variations of this technique. However, the technique is generally divided into two major categories, i.e. arc stud welding and capacitor discharge stud welding. In arc stud welding, current is passed through the stud to be welded while in contact with the workpiece and then lifted to create an arc between the stud and workpiece. After sufficient time passes to permit the arc to create melting of the stud and workpiece, the stud is returned to the workpiece into the molten pool of metal. In this mode of welding, an arc shield is placed around the end of the stud and in contact with the workpiece to contain the molten pool of metal to form a weld fillet following solidification of the metal. This technique is used for larger diameter studs or the conventional shear connectors in ranges in excess of 1/2" of diameter.
The arc stud welding technique generally employs only a rounded configuration on the end of the stud to be welded. The current density passing through the end of the stud being welded is relatively low compared to capacitor discharge welding as will be discussed hereinafter and is generally in the area of approximately 5,000 amps per square inch. The weld time for arc stud welding varies depending on the application and the diameter of the stud but it is generally in the area of approximately 0.5 seconds for an average application for welding of a 1/2" stud. Thus, arc stud welding is generally considered to be a longer low current welding process with essentially arc creation resulting in melting of the stud and the workpiece with little or no significant instantaneous disintegration of the end of the stud.
The capacitor discharge stud welding technique differs significantly from the arc stud end welding technique in many ways. In capacitor discharge stud welding, the power source is not a continuous power source as in arc stud welding but is a stored energy source such as that from a bank of capacitors which have been charged to a predetermined level before initiation of the welding cycle. Additionally, the studs utilized in capacitor discharge stud welding are usually of a small diameter in the range of 1/4" or less and also include a small diameter and length welding tip on the end of the stud. The welding tip serves to space the end of the stud to be welded from the workpiece at the beginning of the welding cycle. Upon initiation of the welding cycle, the readily available energy supply from the energy source such as the capacitors is dumped through the stud at an extremely high current density resulting in complete disintegration and vaporization of the welding tip. The disintegration of the welding tip momentarily leaves the stud spaced from the workpiece while an arc is established between the stud and the workpiece substantially along the entire face of the stud due to the high energy level of the capacitors.
In an average capacitor discharge stud welding environment, the current density passing through the tip of the welding stud is, momentarily, in the range of approximately 10,000,000 amps per square inch. This flash of high density current is substantially instantaneous and the entire weld cycle for an average capacitor discharge welding cycle is approximately 0.003 seconds.
Capacitor discharge welding is used primarily for rather small studs, high production rates and with thin sheet base material which cannot withstand the longer welding and heating cycles of arc stud welding. Additionally, the weld strength of a capacitor discharge weld is somewhat less than that of the arc stud welding technique.
The stud end welding technique, either arc stud welding or capacitor discharge stud welding, has not, heretofore, been successfully used in the welding of studs having two or more ends which require to be welded simultaneously to the base member. There are numerous stud configurations which have two or more ends which are required to be welded and thus cannot utilize the stud end welding technique. Examples of such studs are double ended lifting hooks, handles and hold-down loops all of which must be welded by electric or gas hand welding.
Another major category of stud which includes two or more ends and which must currently be hand welded is the double ended shear connector. A double ended shear connector is a U-shaped metallic member which is welded to an I-beam or the like and is later embedded in concrete lying upon the I-beam to provide a shear interconnection between the concrete slab lying upon the beam and the beam itself.
There are basically two types of shear connectors in use in the industry today. The first kind is the headed shear connector which is an elongate rod like member having one end weldable thereon and an enlarged flanged head at the opposite end. This stud is generally of round configuration and of approximately 3/4 inches in diameter and is capable of being welded by the conventional arc stud welding technique. The other design of shear connector conventionally in use is a generally U-shaped member of rectangular cross section of approximately eight gage thickness and 1 inches wide with a height of approximately 3 inches and a separation of the two legs of approximately 45/8 inches. This class of shear connector may be referred to as the double ended shear connector whereas the single elongate shear connector is generally known as the headed shear connector.
A double ended shear connector of less total metal weight than a headed shear connector can still provide equal or greater shear resistance when welded in place. Thus, the material costs and performance of a double ended shear connector is superior to that of a headed shear connector. However, the double ended shear connector suffers from the disadvantage in that it must be manually welded and has not, heretofore, been capable of being welded by the stud end welding technique. The hand welding technique is more time consuming than the stud end welding technique and, additionally, the skill required to manually weld the double ended shear connector is greater than that required of the operator for the stud end welding technique with the further consequent addition of expense in the trade utilized. Accordingly, there is a real industry need for a method and apparatus to weld double ended shear connectors by the stud end welding technique.
Different techniques have, in the past, been attempted to arrive at apparatus and methods to successfully weld multi-ended studs. The foremost problem encountered with the welding operation of double ended studs is the initiation of an arc on both legs of the stud. If the arc initiates on one leg and nothing is done to enhance initiation on the other leg, the arc will continue to operate on the first leg and the second arc will not be initiated. There are two main reasons for this occurrence. First, the operating arc will cause a large voltage drop from the open circuit voltage. This will make it increasingly difficult to break down the air gap resistance at the unarcing leg. Secondly, the heating of the arcing leg will lower the work function and increase the electron flow at that point, effectively lowering the resistance. These two effects combine to make it extremely difficult to initiate a second arc at the remaining legs once there has been the establishment of a first arc.
The desired solution to this problem of single arc initiation is to initiate arcs on both legs of the stud simultaneously. However, with the conventional stud welding apparatus and method of stud liftoff initiation, it is nearly impossible to maintain equal initial arc gaps. Even a very small difference in gap size works to prevent one arc from starting because the breakdown voltage can be in the order of 1,000 volts/mil. in air. Both legs must leave the base plate surface at exactly the same time or only one arc will occur.
Assuming that dual arcs can be initiated, the second problem encountered is to insure that equal welding takes place on both legs of the stud. Essentially, this means that equal welding currents must be maintained in both legs. Differences in current readily result from such conditions as oxides on the work surface or suttle changes in metal transferred through either arc. Accordingly, it is extremely difficult if not impossible to maintain equal current density in the welding legs and thus, uneven melting occurs with the consequence of one leg of the double ended stud welding not being sufficiently welded.
One attempt made at solving the problem of uneven arcing between the two legs of a double ended stud was to place an insulator between the two legs and apply separate current sources to the welding legs. Such a concept is disclosed in U.S. Pat. No. 2,788,434. This solution suffers the rather critical disadvantage of the two legs of the stud being separated by a weak insulator between the two legs which seriously reduces the strength of the stud.
Another solution tried was to vary the geometry of the ends of the double ended stud by utilizing such configurations as chisel points as well as other configurations such as pointed ends, rounded ends and square ends. In some cases, fluxes were used to help lower the ionization potential and provide a shielding atmosphere. The use of differing stud end configurations and fluxes did not prove successful. In a few cases, two arcs would occur at the two legs. However, one arc was always larger than the other. The smaller arc usually produced no melting of either the stud or the base plate. The lighter arc generally became nothing more than a brief spark which caused slight heating of the stud and the base plate. The utilization of fluxes aided slightly in the process but still did not result in sound commercially acceptable welds. Even compounds that exhibit lower ionization potential and higher electroconductivity than iron, although aiding in establishing arcs on both stud legs, did not effectively equalize or maintain the required dual arcs.
Another approach which has been investigated but found unsuccessful for double ended stud welding is that based upon the principles of arc gap effect and arc initiation and thermal emissivity at elevated temperatures. In this approach, the hypothesis is that, if the arc could be briefly extinguished on the operating leg by removal of the welding power, then the arc could be reignited on the opposite leg by virtue of that being the shortest arc path. It was postulated that, in order for the shorter path effect to dominate, the time that the arc was extinguished would have to be sufficient to allow a complete dissipation of the electron cloud and for the arc atmosphere to cool below levels where thermal ignition effects dominate.
In this approach, the power supply providing the welding current to the double ended welding stud was operated through a controller in a manner such that the welding current was rapidly turned on and off to the welding stud in bursts of energy spaced one from another in the order of 100 milliseconds. In some instances, dual arcing at both legs of the double ended stud were achieved. However, control of positioning the stud with respect to the workpiece was extremely delicate and difficult and the control of the arcing between the two legs was erratic and generally unsatisfactory welds resulted.